1. Field of the Invention
The present invention relates to a fuel injector for an engine.
2. Description of the Related Art
In a known fuel injector, the opening and closing of the nozzle holes is controlled by one end of a needle, and a pressure control chamber is formed between the piston and the other end of the needle. The pressure control chamber is connected to a high pressure fuel source via a fuel passage having a restricted flow area, and the piston is actuated by the piezoelectric element. When the volume of the pressure control chamber is increased due to the movement of the piston, the needle opens the nozzle holes, and when the volume of the pressure control chamber is decreased due to the movement of the piston, the needle closes the nozzle holes (see Japanese Unexamined Patent Publication No. 59-206668).
In this fuel injector, the pressure control chamber is filled with fuel under a high pressure, and when the piezoelectric element is caused to contract and the piston accordingly moved to increase the volume of the pressure control chamber, the pressure of the fuel in the pressure control chamber temporarily becomes low. At this time, the needle opens the nozzle holes, and the pressure of the fuel in the pressure control chamber is increased to the initial high pressure. Conversely, when the piezoelectric element is caused to expand, and the piston accordingly moved to reduce the volume of the pressure control chamber, the pressure of the fuel in the pressure control chamber temporarily becomes high. At this time, the needle closes the nozzle holes, and the pressure of the fuel in the pressure control chamber is decreased to the initial high pressure. Consequently, in this fuel injector, the pressure control chamber is normally filled with fuel under a high pressure, and this high pressure acts continuously on the piezoelectric element via the piston.
Where, however, the fuel injector has a construction such that the pressure of fuel in the pressure control chamber acts on the piezoelectric element, when the pressure of fuel fed into the pressure control chamber via the fuel passage having a restricted flow area is changed, the load acting on the piezoelectric element is changed accordingly, and as a result, when electric power is supplied to the piezoelectric element, the amount of expansion of the piezoelectric element is changed in accordance with a change in the load acting on the piezoelectric element, and thus a problem arises in that it is difficult to precisely control the opening and closing of the needle.